The primary objective of these studies is to determine the feasibility of utilizing the cloned lux gene system isolated on a plasmid expression vector from luminous marine bacteria and expressed in E. coli (or possibly other bacteria) to detect toxic substances in water. It is well known that numerous toxic substances attenuate the luminescence from the marine bacteria, presumably due to the insertion of the toxic materials into the cell membrane and partial disruption of membrane-associated processes. It is not known whether the same processes will occur in E. coli carrying the genetic material that encodes the polypeptides responsible for luminescence. These are numerous advantages to utilization of the cloned gene system in E, coli rather than in the native environment. First, E. coli is not a marine organism and does not require addition of salt to the test water. Second, the system in E. coli is under ready genetic control. Third, we can move the same genetic system in numerous different host organisms for testing for different substances. Luminescence assays are both exquisitely sensitive and fast. It is possible to detect the luminescence from a single bacterial cell with standard equipment. Addition of low levels of toxic substances to glowing cultures of bacteria results in attenuation of light that is apparent within 30 sec. Therefore, if the assay is feasible in E. coli or other nonmarine bacterium, it should be readily adaptable to rapid broad spectrum screening of water supplies for toxic substances, i.e., water from waste treatment facilities could be screened on a regular basis.